The basic objectives of this research project are to identify and characterize local synaptic circuits between pyramidal cells in rat hippocampus. We are using simultaneous intracellular recordings from pairs of pyramidal cells to study their interactions in the hippocampal slice preparation. Our strategy is to stimulate a single pyramidal cell and record postsynaptic response from another pyramidal cell. Our preliminary results indicate that interactions between pyramidal cells involve recurrent inhibition, recurrent excitation and probably electrotonic coupling. These pathways will be identified and characterized by determining synaptic delay, duration of postsynaptic responses, and whether the interactions are reciprocal. Plasticity of transmission through each pathway will be examined during spike bursts and repetitive activity to determine the optimum spike pattern and whether potentiation and/or depression exists for each local circuit. Intracellular staining will be used to study the anatomical relationships between the neurons involved in these local circuits. Dye-coupling with the fluorescent dye, Lucifer Yellow CH, will be analyzed to assess electrotonic coupling. This research is aimed at providing an understanding of the intrinsic synaptic processing of pyramidal cells in the hippocampus and may ultimately lead to further insights about their postulated roles in learning and memory and the spread of epileptic seizure activity.